Hardfacing is one of the surfacing methods most widely employed in nuclear valve manufacturing industries to deposit, for example, cobalt-based Stellite brand alloys to enhance specific properties such as wear and corrosion resistance of valve components such as discs, wedges, seats and seat rings. The most common hardfacing techniques employed are oxyacetylene gas welding (OAW), gas tungsten arc welding (GTAW) or tungsten inert gas welding (TIG), gas metal arc welding (GMAW) or metal inert gas welding (MIG), submerged arc welding (SAW), and plasma transferred arc welding (PTA). Among them, the most important differences lie in the welding efficiency and the weld dilution rates. The different hardfacing processes can generate weld deposit thickness between 0.080 inch (2 mm) and 0.800 inch (20 mm). The deposition rate can vary from 0.5 to 20 kg/h. The fusion zone and heat affected zone can be between 0.020 inch (0.5 mm) and 0.400 inch (10 mm).
Dilution is an important parameter in the deposition of weld overlays as it defines the degree to which the substrate material has mixed with the overlay material. This mixing directly affects the composition of the resulting surface layer, along with its microstructure, wear and corrosion performance. As Fe from a steel substrate mixes into an overlay, the Fe dilutes the composition of the overlay, and thereby negatively affects its properties. For example, FIG. 1 shows that the hot hardness of cobalt alloy weld overlays drops off sharply as Fe content increases up to 10 wt %. Each line represents hardness measured at different temperatures 204° C., 427° C., 538° C., 649° C., and 760° C. At higher temperatures this effect is much more pronounced. It is therefore desirable to control the Fe dilution less than 5 wt % to maintain the hot hardness and therefore the wear resistance of cobalt alloy overlays. FIG. 2 shows that adhesion wear resistance and abrasion wear resistance are negatively affected by Fe dilution. The right vertical axis shows the units for adhesion wear resistance in cubic millimeters volume loss and the left vertical axis shows the abrasion wear resistance in cubic millimeters volume loss.
Dilution can be measured by chemical analysis of the Fe profiles, which represents actual dilution in the hardfacing layer. Dilution levels vary widely and depend on a large number of factors, however typical dilution levels of various hardfacing technologies can be stated as follows for illustration: OAW—1%-10%, TIG—15%-20%, MIG—15%-25%, SAW—10%-60%, PTA—5%-30%. The higher heat input required to build up a thick single layer deposit tends to result in more dilution in the deposited layer and distortion of the base structure. Maintaining dilution down in the range between 10% and 15% is generally considered optimum in terms of bond integrity and hardfacing layer integrity. Unfortunately, most welding processes have considerably greater dilution.
For valves in critical applications such as severe service conditions in nuclear, petrochemical, chemical processing, and mining applications, the plasma transferred arc welding (PTA) process is usually used and multiple hardfacing layers are generally applied in order to control the dilution to be below 5%. A first layer typically has high dilution, but being a cobalt-based alloy with Fe diluted therein, it has less Fe than the substrate. Then a second layer, being a cobalt-based layer on a cobalt-based layer diluted with Fe, will have less Fe than the first layer. Then a third layer on top of the second layer will have even less Fe than the second layer. So the Fe content of the outermost layer is lower than the Fe content of the underlying layers and the substrate. So by applying multiple layers, an outermost layer has been provided with reduced Fe content. Theoretically, if the dilution in the first layer is about 30%, the dilution in the second layer may be reduced to about 9%, and to about 2.7% in the third layer. While effective to achieving satisfactorily low Fe levels in the outermost layer encountering the wear and corrosion service conditions, applying multiple layers and thick deposits have a high cost in terms of hardfacing materials and labor. Multiple layers and thick deposits also introduce fusion defects and have a tendency to yield deposits containing cracks.
Some nuclear valves are made from low carbon stainless steels or corrosion resistant nickel alloys. Preferential attack in the fusion zone may occur which may be attributed to a higher than normal level of carbon, resulting from carbon diffusion, during the hardfacing process. The corrosion resistance of the fusion zone therefore may be compromised by excessive dilution of Fe and C near the fusion line.
U.S. Pat. No. 4,521,664 to Miller discloses a process and apparatus for surfacing with high deposition and dilution levels less than 30%. However, high deposit rates are generally accomplished by the use of high heat input which invariably leads to high dilution. It is difficult to achieve high deposit rates and to maintain low dilution simultaneously.
U.S. Pat. No. 4,686,348 to Johns et al. discloses a method for hardfacing valves to reduce the dilution by valve head material. The method involves machining a groove/recess with a special cross-sectional size and/or shape.
U.S. Pat. No. 5,002,839 to Qureshi et al. discloses the use of a buffering layer of austenitic stainless under a cobalt-based layer in processes for manufacturing and repairing valve seats.
U.S. Pat. No. 6,858,813 to Keller et al. discloses an alternating current MIG process which can produce weld overlay with a thickness range from 0.030 inches to 0.100 inches thick, with dilution rates of less than 20%.
U.S. Pat. No. 6,861,612 to Bolton discloses methods for using a laser beam to apply wear-reducing material to tool joints and with reduced dilution of the base metal into the applied wear-reducing material. However, S. Sun et al. found that high dilution can result from laser cladding of cobalt-based alloys. Correlation between Melt Pool Temperature and Clad Formation in Pulsed and Continuous Wave Nd:YAG Laser Cladding of Stellite 6, Proceedings of the 1st Pacific International Conference on Application of Lasers and Optics 2004. Therefore, the melt pool temperature and laser power must be strictly controlled to control the dilution which imposes practical difficulties.
D. Raghu et al. disclose in PTA Proves Its Worth in High-Volume Hardfacing Jobs, Welding Journal, February 1996, pp 34-40, that for PTA welding, dilution can be controlled to about 5 to 7% in most cases when the heat input is precisely controlled. In contrast, other forms of welding deposits can have up to 50% dilution and inconsistent wear resisting properties of the overlay through the weld deposit.
U.S. Pat. No. 6,385,847 to Larson et al. points out that the PTA offers several advantages such as controllable heat source and lower energy consumption which can provide finer microstructure and narrower heat affected zones; versatility for powders and different raw materials; higher volume production capability; and minimum raw material waste. However, the current PTA processes usually operate at such a high temperature that in some valve applications the torch burns through the valve from the seat facing groove to the valve combustion face on the valve head. One solution might be to add additional stock material to the combustion face to act as a heat sink. However, that option adds to the cost of manufacturing due to the extra machining required to remove the material afterwards as well as the cost of the material itself, as a waste material.
The prior art methods therefore provide inconsistent results in terms of dilution reduction. And using current prior art processes, it is necessary to closely control parameters, and even then it is difficult to achieve a hardfacing deposit with less than 5% dilution.